A disc drive is a rotating magnetic storage device capable of storing data which may by accessed at high speeds. This device includes a rotating, flat magnetic surface onto which data can be recorded and later retrieved, usually in milliseconds because of the high rotational rate of the disc and the fast radial accessing capability of the head positioner. The head positioner of the disc drive assembly functions to locate and maintain the magnetic read/write heads at a commanded track position above the disc, and this head positioner also provides precision support and stability which is necessary for the heads to function properly. Most disc drive designs are based upon the moving head principle as opposed to the faster, but more costly, "head per track" concept. Usually more than one disc or disc surface is used per drive, and in this case one or two seperate heads are used for accessing each disc surface. The heads for each surface are mounted on aligned arms of the positioner such that they all move in unison. The resulting head positioner structure looks like a comb and is often referred to the "head comb". The head comb is typically aligned and moved by a support and motor structure which is operative with either linear or rotary motion.
When a head positioner in a disc drive performs a very fast seek, many resonant or ringing modes of vibration can be excited and unless properly damped, will tend to ring for long periods of time, typically on the order of 100 miliseconds or more. Often the amplitude of these modes of vibration at the magnetic head position is a large percentage of the width of a data track on the magnetic disc. If this is the case, data reading or writing will be disabled until the ringing amplitude drops to a value sufficiently low to permit accurate reading or writing.
Disk drives having a high track density typically employ positioning servo schemes in order to ensure accurate and fast track locating by the magnetic heads. However, the accuracy and speed of such servo systems are highly dependant upon the mechanical system resonances. A servo system is a negative feedback control system in which part of the system is mechanical. An ideal control loop for such a servo system feeds the output signal of the system back to the input of the system and out of phase with the input thereto to force the system to exhibit a unity gain transfer function. When the servo system mechanics are introduced, mechanical resonances produce output signals greater than the input signals as well as produce shifted output signal phase relationships relative to the input signal.
Some compensation is customarily introduced into the servo control loop to provide improved stability with the mechanical system. But in spite of this compensation, mechanical resonances of the above type produced by head positioner vibrational resonances often are large enough and provide a phase shift of sufficient magnitude to create a positive feedback for the system at greater than unity gain. When this happens, the servo loop becomes unstable at that resonance frequency.
In the low performance disc drive industry, one practice has been to employ rotational dampers on stepper motors which ar used to drive head positioners for the disc drives. These dampers are used to increase the stability of the stepper motor per se. However, as presently known, these damping techniques of the prior art are not directed to the damping of vibrational resonances of an entire head positioner used in high performance track following servo disc drive actuators, as in the case with the present invention.